Chemistry of Petroleum 1: What Are Hydrocarbons?

What You Need


  • Bag of large marshmallows
  • Bag of raisins
  • Box of toothpicks
Chemistry of Petroleum 1: What Are Hydrocarbons?


To introduce students to organic chemistry and the structure of hydrocarbons.


This lesson is part of the Energy in a High-Tech World Project, which examines the science behind energy. Energy in a High-Tech World is developed by AAAS and funded by the American Petroleum Institute. For more lessons, activities, and interactives that take a closer look at the science behind energy, be sure to check out the Energy in a High-Tech World Project page.

This lesson is the first in a series of four about the chemistry of petroleum, intended for upper-level chemistry students in the 11th and 12th grades. The goal of these lessons is to introduce high-school students to the use of oil as an energy source in today’s high-tech world. In the Chemistry of Petroleum 1: What Are Hydrocarbons? students will explore hydrocarbons, the molecular basis of petroleum, and learn to distinguish between organic and inorganic compounds.

In the Chemistry of Petroleum 2: What Happens When Hydrocarbons Burn? students will examine the varying amounts of energy produced by the combustion of different hydrocarbons.

In the Chemistry of Petroleum 3: Distillation of Hydrocarbons, students will be introduced to the distillation and treatment processes by which petroleum is refined to produce useful fuel oils.

The Chemistry of Petroleum 4: Treatment of Hydrocarbons will help students explore the chemical treatment processes by which distilled petroleum fractions are converted to produce useful fuel oils.

The study of organic chemistry, and more specifically hydrocarbons, is an excellent way for students to learn about the structure of matter. As Science for All Americans indicates, the physical world seems to have a stunningly varied array of materials. These materials differ greatly in shape, density, flexibility, texture, toughness, and color; in their ability to give off, absorb, bend, or reflect light; in what form they take at different temperatures; in their responses to each other; and in hundreds of other ways. Yet, in spite of appearances, everything is really made up of a relatively few kinds of basic material combined in various ways. As it turns out, about 100 such materials—the chemical elements—are now known to exist, and only a few of them are abundant in the universe. Of these chemical elements, carbon is one of the most important to life. (Science for All Americans, p. 46.)

Through the process of photosynthesis, plants harvest energy from sunlight and use it to combine carbon and hydrogen into high-energy compounds called “hydrocarbons.” Carbon, in the form of hydrocarbons and other organic compounds, is passed from organism to organism through the food web and eventually recycled by decomposers back to nutrients usable by plants. Some carbon-based plants and animals that died millions of years ago in the oceans sank to the bottom, where their hydrocarbons were buried under sediment and mud. The intense pressure and heat (125 - 300º C) over millions of years on these deeply buried hydrocarbons produced petroleum, which today is refined to produce a myriad of carbon-based fuels and materials for human use.

These lessons require that students have some prerequisite knowledge of the basics of atoms. For example, students should be familiar with the structure of the atom as well as the organization of electrons into shells. An understanding of valence electrons is necessary for students to understand why the carbon atom forms four covalent bonds. Some basic information about atoms can be found at The Atom.

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Begin the lesson by telling students that they will begin a unit on organic chemistry; that is, they will explore compounds that are “organic.”

Ask students: What do you think of when you hear the word “organic”?

Write down the students’ thoughts on the board. It is likely that some of their thoughts will revolve around agricultural methods or farming techniques. As students contribute their ideas, note how a single word can mean different things to different people.

It is important to begin with this activity as it will allow you to determine your students’ misconceptions of the term “organic.” This activity can be repeated at the end of all the lessons to assess students’ understanding. Alternatively, students’ initial thoughts can be recorded and re-visited at the end of this lesson. This will allow students to correct their own misconceptions and to evaluate their own understanding of the topic over time.

Have students use their What Are Hydrocarbons? student esheet to go to and read What is Organic Chemistry? and Organic Chemistry: An Introduction. They should answer the questions found on the What Are Hydrocarbons? student sheet. You can find answers to the questions on the teacher sheet.

After they have read the article, discuss what the study of organic chemistry is, ensuring that students understand that the term organic has a different connotation today when used for food, natural items, and synthetic items.

  • We know that the term “organic” means different things to different people. What does “organic” mean to chemists?
    (It means carbon-based.)
  • What makes an organic compound different from an inorganic compound?
    (Organic compounds contain carbon, whereas inorganic compounds do not.)
  • Why is organic chemistry important to studying living organisms?
    (It is important because living organisms are carbon-based, meaning that they are made of organic compounds.)
  • What are some examples of organic compounds given in the article?
    (Examples include petroleum, plastics, vitamins, sugar, and gasoline.)


As a demonstration, draw or write the chemical structure of glucose (C6H12O6) and ask students:

  • Is this compound organic or inorganic?
    (It is organic.)
  • How do we know?
    (It has carbon.)

Tell students that the chemical structure on the board is that of the sugar, glucose. Repeat this activity for a number of different common organic and inorganic compounds. For example:

  • C5H4N4O3 – uric acid (organic)
  • H2O – water (inorganic)
  • C21H22N2O2 – strychnine (organic)
  • C17H19NO3 – morphine (organic)
  • NaCl – salt (inorganic)
  • C19H28O2 – testosterone (organic)
  • CH4 – methane (organic)

Ask students:

  • Imagine the organic compounds that you just identified were made in a lab by a scientist. Would she, the scientist, still classify them as organic compounds even though they are not naturally found?
  • Why would she still identify them as organic even though they are synthetic, made by humans?
    (The compounds contain carbon and that is the only characteristic that is important in classifying compounds as organic. A compound is classified as organic based on it containing carbon, not on whether it is naturally found or synthetic.)

Circle or point to the methane chemical formula (CH4). Tell students that this particular organic compound only has two types of atoms—carbon and hydrogen. There are some organic compounds that only contain atoms of carbon and atoms of hydrogen. These compounds are categorized as hydrocarbons. Go through the other organic compounds listed on the board, clarifying that compounds can be organic and not hydrocarbons. In contrast, all hydrocarbons are organic because they contain carbon. It may help students to chart out or diagram the relationship between inorganic compounds, organic compounds, and hydrocarbons.

Review the basic chemistry of carbon with students. The following questions may be asked:

  • How many electrons does a carbon atom have?
    (It has six.)
  • Of those six electrons, how many are valence electrons?
    (Four are valence electrons.)
  • How many more electrons does carbon need to complete its valence shell?
    (It needs four more.)
  • How many bonds can it make with other atoms?
    (It can make up to four bonds.)
  • How many electrons does hydrogen have?
    (It has one.)
  • How many more electrons does hydrogen need to complete its valence shell?
    (It needs one more.)
  • How many bonds does hydrogen make with other atoms?
    (It makes one bond.)

Provide students with large marshmallows (representing carbon), raisins (representing hydrogen), and toothpicks (representing a covalent bond). Ask students to use the items to visualize methane’s structure. Students should have a marshmallow with four toothpicks, each connected to a raisin. Ask students:

  • Count the number of electrons that your marshmallow carbon now has by sharing with the hydrogens.
    (It has eight electrons.)
  • Is its valence shell complete?
  • Count the number of electrons that your raisin hydrogens now each have by sharing with carbon.
    (They have two electrons.)
  • Is its valence shell complete?

Ask two students for their methane models and hold them next to each other. Ask students how the two compounds can join together to form one. Have students work with a partner to form ethane. Ask students to count the number of shared electrons that their carbons and hydrogens each have to confirm that they have the appropriate number of bonds (e.g., that no hydrogen has two bonds, giving it four shared electrons, rather than two).

Ask students:

  • What is the chemical structure of the ethane compound you just made with your partner?
    (It is C2H6.)
  • Is this a hydrocarbon?
  • Is it organic?

Remind students that carbon can form double bonds. Using one of the student’s ethane models, add a second toothpick between the marshmallow carbons. Ask students to work in pairs again to re-organize their models so that the carbons and hydrogens have the correct number of shared electrons. Students should remove a raisin hydrogen from each of the carbons. Tell students that the ethane compound has changed; it now has a double bond and its chemical structure has changed. Ask students:

  • What is the chemical structure of this new compound?
    (It is C2H4.)
  • Is this still an organic hydrocarbon?
  • What have we done to the ethane molecule?
    (We have added a double bond.)
  • If the chemical formula for the compound has changed, is it still ethane?

Tell students that in organic chemistry, there is a way of naming hydrocarbons so that we know if they have a double or single bond between any of the carbons. This molecule is ethene. The –ene suffix indicates a double bond, whereas the –ane suffix indicates a single bond. Collectively, hydrocarbons with a double bond between two carbons are called alkenes and hydrocarbons with single bonds between all the carbons are called alkanes.

Provide each student with another marshmallow and ask them to make an “alkane” with three carbons. After they are done, have them check their model with a partner. Ask students:

  • Why do we know that you have alkanes and not alkenes?
    (There are only single bonds between the three carbons.)
  • What is the chemical formula for your compound?

Refer students to the prefix chart of hydrocarbons found at Organic Chemistry: An Introduction. Ask students to name their compound based on the chart. Students should determine that their models represent propane. Ask students to take their propane and make it into propene. Next, ask students to make a triple bond between two of the carbons and alter their models accordingly. Tell students that this is neither propane nor propene; rather, it is propyne. The –yne suffix indicates a triple bond between two carbon atoms of a hydrocarbon.

Go through the prefix chart with students, ensuring that students can say each name correctly as an alkane, alkene, and alkyne. Ask students:

  • Have you heard of some of these hydrocarbons before?
    (Answers will vary, but common hydrocarbons are propane, butane, and methane.)
  • Is it possible to have more than ten carbon atoms joined together as a hydrocarbon?


Using their marshmallows, raisins, and toothpicks, ask students to construct these hydrocarbons:

  1. Ethane
  2. Butyne (students should have two types)
  3. Hexane
  4. Heptene (students should have three types)

Ask students why organic chemists say that there are almost an infinite number of different hydrocarbons. Students should indicate that carbons can be linked together continuously, making long chains. They also can contain double and triple bonds in various locations. As the hydrocarbon grows in size, the number of double and triple bond possibilities grows. To emphasize this, ask students to make an octyne compound that also contains one double bond. Students should realize that the compound can have more than one double or triple bond in different locations.

Ask students:

  • Imagine we linked up all of the marshmallow carbons into one long chain with single bonds. Would that be a flimsy or strong structure?
    (It would be flimsy.)
  • Why?
    (The single bonds can break easily.)
  • How could we make that structure really strong without removing any of the carbons?
    (We could make it really strong by adding double and triple bonds in as many places as possible.)
  • Hydrocarbons with up to 18 carbons attached together are usually liquid, but hydrocarbons with more than 18 carbons are usually solid. Why do you think that is?
    (Smaller hydrocarbons will have less double and triple bonds and therefore, less rigidity, than the larger hydrocarbons which contain double and triple bonds to stabilize their structures.)


Follow this lesson with the next three lessons in the chemistry of petroleum series:

Have students work in teams to investigate the use of one alkane historically and today. A good starting point for students to begin their research is Summary of the Properties and Uses of Hydrocarbons.

The presence of hydrocarbons on other planets is a telling sign for the possibility of life. Have students read a 2007 article about the discovery of hydrocarbons on Saturn’s moon, Hyperion called Hydrocarbons, Necessary for Life, Found on Saturn’s Moon Hyperion.

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Lesson Details

Grades Themes Type Project 2061 Benchmarks National Science Standards State Standards